Covalent Organic Frameworks (COFs) are three dimensional crystalline materials prepared by linking lighter elements (e.g., B, C, N, O) via covalent bonds in a periodic manner. COFs are typically synthesized and subsequently crystallized by means of reversible condensation reactions/covalent bond formation reactions like boronic acid trimerisation, boronate ester formation and Schiff base reaction. Structurally, COFs are closely related to metal-organic frameworks (MOFs), where coordination bonds link metal ions and organic struts. Metal-organic frameworks (MOFs) can facilitate proton conduction by accommodating guest molecules, such as water and imidazole, in well-defined pores or integrating functional acidic groups onto the channel walls. Although COFs have shown excellent promise as semiconductive device, sensors, in gas storage and in separation, but proton conductivity in COFs are still unprecedented. Structurally, COFs are closely related to metal-organic frameworks (MOFs), where coordination bonds link metal ions and organic struts. Although COFs have shown excellent promise as semiconductive devices, sensors and in gas storage and separation proton conductivity in COFs is still unexplored.
In recent years, proton conducting materials have gathered remarkable interest among researchers due to their application in fuel cells, sensors and electronic devices, refer, (a) Mauritz, K. A.; Moore, R. B. Chem. Rev. 2004, 104, 4535 and (b). Hickner, M.; Ghassemi, H.; Kim, Y. S.; Einsla, B. R.; McGrath, J. E. Chem. Rev. 2004, 104, 4587. Nafion based proton conducting membranes are considered as the benchmark in this field which exhibit high proton conductivity (ca. 10−1 Scm−1) at moderate temperature (60-80° C.) under high relative humidity (98% RH), refer Paddison, S. J. Annu. Rev. Mater. Res. 2003, 33, 289. However, high cost of Nafion, (perfluorinated membranes) with less efficiency at fuel cell operating temperature (120° C.) always encouraged researchers to search for alternative materials. In this context, MOFs with loaded carrier molecules (e.g., imidazole, triazole, mineral acids) has been envisaged for high temperature proton conduction applications and several references are available for the same. However, these MOFs suffer poor hydrolytic stability with very low pH tolerance of the occluded guests. As a result, rupture of the coordination bonds and the framework backbone occurs, which limits its applicability in fuel cell operating conditions. In addition, high gravimetric weight of MOF, difficulty in forming compact membrane and its stability at higher temperatures are necessary to consider for future development of proton conducting materials.
Article titled “Proton-conducting membranes based on benzimidazole polymers for high-temperature PEM fuel cells. A chemical quest” by J A Asensio et al. published in Chem. Soc. Rev., 2010, 39, pp 3210-3239 reports Proton-conducting membranes based on benzimidazole polymers for high-temperature PEM fuel cells. They also reports Acid-impregnated polybenzimdazole type membranes with high thermal stability after PA loading. However, lack of crystallinity of polymeric membranes results in limitations such as multidirectional hopping of protons which affect the proton transport rate and insufficient mechanistic insight of the transport mechanism limit further improvement of the material.
Article titled “Chemically stable multilayered covalent organic nanosheets from covalent organic frameworks via mechanical delamination” by S Chandra et al. published in J. Am. Chem. Soc., 2013, 135 (47), pp 17853-17861 reports a series of five thermally and chemically stable functionalized covalent organic frameworks (COFs), namely, TpPa-NO2, TpPa-F4, TpBD-(NO2)2, TpBD-Me2, and TpBD-(OMe)2 synthesized by solvothermal aldehyde-amine Schiff base condensation reaction.
Article titled “Construction of crystalline 2d covalent organic frameworks with remarkable chemical (acid/base) stability via a combined reversible and irreversible route” by S Kandambeth et al. published in J. Am. Chem. Soc., 2012, 134 (48), pp 19524-19527 reports two new chemically stable [acid and base] 2D crystalline covalent organic frameworks (COFs) (TpPa-1 and TpPa-2) synthesized using combined reversible and irreversible organic reactions. Synthesis of TpPa-1 and TpPa-2 COFs was done by the Schiff base reactions of 1,3,5-triformylphloroglucinol (Tp) with p-phenylenediamine (Pa-1) and 2,5-dimethyl-p-phenylenediamine (Pa-2), respectively, in 1:1 mesitylene/dioxane.
PCT application No. 2014057504 discloses covalent organic frameworks (COFs) which exhibit stability towards acidic, basic and neutral conditions and process for the synthesis thereof. Also, the invention provides an environmentally-friendly mechanochemical/solvothermal process for the construction of stable covalent organic frameworks (COFs) efficiently at a faster rate and in high yield.
Article titled “Imparting High Proton Conductivity to a Metal-Organic Framework Material by Controlled Acid Impregnation” by V G Ponomareva et al. published in J. Am. Chem. Soc., 2012, 134 (38), pp 15640-15643 reports the impregnation of the mesoporous metal-organic framework (MOF) MIL-101 by nonvolatile acids H2SO4 and H3PO4 to affords solid materials with potent proton-conducting properties at moderate temperatures, which is critically important for the proper function of on-board automobile fuel cells.
Article titled “Proton conductivity of phosphoric acid doped polybenzimidazole and its composites with inorganic proton conductors” by R He et al. published in Journal of Membrane Science, 1 Dec. 2003, 226 (1-2), pp 169-184 reports phosphoric acid doped polybenzimidazole (PBI) and PBI composite membranes. They also reports that the conductivity of phosphoric acid doped PBI and PBI composite membranes is dependent on the acid doping level, relative humidity (RH) and temperature.
The qualities of COFs such as light weight in nature, wide variety of functionality, thermal stability and membrane processability like polymers, ensure its sustainability in harsh fuel cell operating conditions and high degree of internal ordering like MOFs, enable loading and transport of proton conducting substrates. Despite the above promising features, COFs have never been tested for proton conduction due to their instability in ambient humidity conditions.
Therefore, there is an unmet need in the art to develop COFs with greater stability under ambient conditions, so as to increase their applications in proton conduction.